Fossil fuels are non-renewable resources, and there have been continued efforts towards developing sustainable alternatives such as biofuels. Bioethanol, for example, can be used as motor fuels alone or as a high-octane-number and CO-reducing additive to gasoline. However, a significant challenge exists for the production of fuel ethanol via biomass conversion, which is related to the separation of the desired ethanol from its relatively dilute aqueous solution. This separation is traditionally carried out by distillation, which consumes a large amount of energy. Although the heat input required for distillation can come from sources other than the product fuel (e.g., burning feed residues) in certain cases (e.g., sugarcane-based technology), it is not possible for all ethanol production technologies nor desirable if the feed residues can find other uses (e.g., in a lignocellulosic ethanol plant). In addition, if anhydrous ethanol with concentrations exceeding the ethanol-water azeotrope concentration is required, then a more energy-intensive azeotropic distillation step is conventionally needed.
Zeolites play numerous important roles in modern petroleum refineries and have the potential to advance the production of fuels and chemical feedstocks from renewable resources. The performance of a zeolite as separation medium can depend on its framework structure and the type or location of active sites. To date, 213 framework types have been synthesized and >330,000 thermodynamically accessible zeolite structures have been predicted. Hence, identification of optimal zeolites for a given application from the large number of candidate structures is attractive for accelerating the pace of materials discovery.
Separating ethanol from its aqueous solution, a process essential for biofuel production, has traditionally relied on energy-intensive distillation. Nearly defect-free silicalite-1, an all-silica zeolite with the framework type MFI, has been proposed as an effective sorbent and membrane for this separation.
Certain porous materials can be used to separate a component from a mixture by selective adsorption or transport. For example, K+-exchanged aluminosilicates of the LTA type (Zeolite 3A) strongly adsorb water while not allowing ethanol to pass through. Processes utilizing LTA-type zeolites to further dry ethanol after the primary distillation, for example to replace azeotropic distillation, are disclosed in U.S. Pat. Nos. 2,137,605, 4,273,621, 4,407,662, 4,465,875, and 2010/0081851.
Adsorptive separation processes aimed to replace primary distillation have also been reported. U.S. Pat. No. 4,277,635 discloses a liquid phase adsorption technique using MFI-type silicalite as the sorbent to extract ethanol. The residual stream can be displaced by highly concentrated ethanol under laminar flow condition, which according to U.S. Pat. No. 4,277,635 results in substantially no intermixing and the product is of gasohol quality. U.S. Pat. No. 4,343,623 further improves the hydrophobicity of the silicalite sorbent via esterification of surface hydrophilic sites. U.S. Pat. No. 4,382,001 discloses the use of a hydrophobic activated carbon as the sorbent and a compound such as isooctane as the desorbent so that the product can be directly blended into gasoline. Canadian 1195258 discloses a separation process also using silicalite as sorbent, but based on vapor phase adsorption. Carrying out adsorption in the vapor phase for a stripping gas sufficiently rich in ethanol improves ethanol recovery and avoids the possibility of solids contained in the dilute solution plugging the porous sorbent. U.S. Pat. Nos. 4,061,724 and 4,073,865 describe the synthesis of silicalite and F-silicalite.